nixos/sshd: update kexAlgorithms, fix links
nixos/sshd: update kexAlgorithms, fix links

The `curve25519-sha256` key exchange method is defined in RFC 8731 that
is identical to curve25519-sha256@libssh.org. OpenSSH supports the
method since version 7.4, released on 2016-12-19. It is literally a
violation of the "both in Secure Secure Shell and Mozilla guidelines"
rule, but it provides essentially the same but a future-proof default.

Also, links to the Mozilla OpenSSH guidelines are updated to refer to
the current place.

Signed-off-by: Masanori Ogino <9fd08c6158f4e2c4695b3924e22cb1a790721236@users.noreply.github.com>

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